1. Field of the Invention
The present invention relates to hydraulic systems for working machines, and in particular, to hydraulic systems for working machines provided with a pair of track devices and a dozer device.
2. Description of the Background Art
Working machines provided with a pair of track devices and a dozer device, such as, for example, the working machine disclosed in Japanese Laid-Open Patent Publication No. 2006-161510, have been proposed to date.
The working machine according to this reference is provided with a swivel base configured to swivel about a vertical axis on a track body which is equipped with a dozer device on a front part thereof, and a digging device furnished on a front part of the swivel base.
The track body includes a pair of left and right crawler track devices which are driven by a track drive motor, and the dozer device includes a blade which is moved up and down by means of a dozer cylinder.
The swivel base is swiveled by a swivel motor.
A swing bracket configured to be swung left and right about the vertical axis is provided on the front part of the swivel base. The swing bracket is swung left and right by a swing cylinder.
The digging device includes a boom pivotally connected to the swing bracket, an arm pivotally connected to the boom, and a bucket pivotally connected to the arm. The boom is swung by a boom cylinder, the arm is swung by an arm cylinder, and the bucket is swung by a bucket cylinder.
The track drive motor and the swivel motor are each constituted by a hydraulic motor, and the dozer cylinder, the swing cylinder, the boom cylinder, the arm cylinder, and the bucket cylinder are each constituted by a hydraulic cylinder.
The working machine is equipped with a hydraulic system including a load sensing system.
The hydraulic system includes: a first pump and a second pump whose discharge flow rates can be controlled; a third pump whose discharge flow rate is not controlled; a flow control section for controlling the discharge flow rates of the first and second pumps; and a pilot pressure valve for switching the direction of discharged fluid from the first pump and the second pump.
The pilot pressure valve is switchable between an independent position in which hydraulic fluid from the first pump and hydraulic fluid from the second pump are supplied independently to left and right track control valves, respectively, and a merging position in which the hydraulic fluid from the first pump and the hydraulic fluid from the second pump are merged and then supplied to a boom control valve, an arm control valve, a bucket control valve, and a swing control valve. The pilot pressure valve is switched to the independent position in a running state of the working machine, and to the merging position in a non-running state of the working machine.
Discharged fluid from the third pump can be supplied to a swivel control valve and a dozer control valve in the non-running state, and can be supplied additionally to the boom control valve, the arm control valve, the bucket control valve, and the swing control valve in the running state.
Each of the boom control valve, the arm control valve, the bucket control valve, and the swing control valve includes a direction switching valve for switching the direction of the hydraulic fluid with respect to a corresponding hydraulic actuator to be controlled, and in addition, includes a pressure compensation valve configured to adjust, when more than one of the hydraulic actuators under control of these control valves are concurrently operated, the loads among the concurrently operated hydraulic actuators.
The pressure compensation valve in a corresponding control valve having a lower load pressure generates a pressure loss equivalent to the differential pressure between the control valve having a lower load pressure and a control valve having the maximum load pressure, thereby realizing a flow rate corresponding to the extent to which the spool in the corresponding control valve is moved, irrespective of the magnitude of the load applied.
Further, in the hydraulic system, in a case where more than one of the boom cylinder, the arm cylinder, the bucket cylinder, and the swing cylinder are concurrently operated in the non-running state, the maximum load pressure of the load pressures acting on the operated hydraulic actuators (hereinafter referred to as PLS signal pressure) is transmitted to the flow control section. Also, the discharge pressure of merged fluid of the discharged fluid from the first pump and the discharged fluid from the second pump (hereinafter referred to as PPS signal pressure) is transmitted to the flow control section. Then, the flow control section automatically controls the discharge flow rates of the first pump and the second pump such that “PPS signal pressure—PLS signal pressure” is maintained at a set value.
In an actual job, with respect to earthwork using the dozer device (blade), the blade is often moved while the working machine is running (for example, in a case where gravel or dry sand is spread by using the blade, the blade is often moved up and down while the working machine is running, so that the gravel or the dry sand is spread evenly. In paving work or the like, in order to grade the surface, the blade is manipulated in order to adjust the tilt of the working machine while the working machine is running).
In the working machine disclosed in the abovementioned reference, in a case where the dozer device is operated while the working machine is running, one of the left and the right track drive motors is driven by means of the discharged fluid from the first pump, and the other of the left and the right track drive motors is driven by means of the discharged fluid from the second pump. In addition, the dozer device is driven by the third pump in order to ensure the straightness of its running and the turning performance of the track device. However, when the dozer cylinder or the like is not operated, the third pump is driven to no avail. This results in low system efficiency.
A circuit configuration that allows the hydraulic actuators included in the working machine (backhoe) to be operated only by the first and the second pumps would eliminate the third pump, and improve the system efficiency. However, in a case where the dozer device is driven when the backhoe is running, if independently one of the left and the right track drive motors is driven by means of the discharged fluid from the first pump, and the other of the left and the right track drive motors is driven by means of the discharged fluid from the second pump, and further the dozer cylinder is driven by means of the discharged fluid from one of the above hydraulic pumps, some of the discharged fluid from the one of the above hydraulic pumps is drawn by the dozer cylinder. This results in a poor straightness in running and extremely poor turning performance.
Therefore, usually, in the case of the circuit configuration that allows the hydraulic actuators included in the backhoe to be driven by the first and the second pumps, the following circuit configuration is employed. That is, in a case where the backhoe just runs, the hydraulic fluid from the first pump and the hydraulic fluid from the second pump are independently supplied to the left and the right track control valves, respectively, and in a case where the dozer device is driven when the backhoe is running, the discharged fluid from the first pump and the discharged fluid from the second pump are merged and then supplied to the left and the right track control valves and the dozer control valve.
However, with this circuit configuration, the independence between the left run and the right run cannot be maintained when the dozer device is driven. Therefore, there remains a problem that the system exhibits poor turning performance.
Therefore, a hydraulic system is desired that is based on a hydraulic system in which the track drive motors, the dozer cylinder, and other hydraulic actuators included in the working machine are driven by use of hydraulic fluid from two independent hydraulic-fluid discharge ports, and that can ensure an independent circuit configuration in which even when the dozer device is operated while the working machine is running, the hydraulic fluid from one hydraulic-fluid discharge port is supplied to one track control valve, and, independently, the hydraulic fluid from the other hydraulic-fluid discharge port is supplied to the other track control valve.